2stroke Engine vs 4 Stroke Engine

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Reed valve engine

The reed valve engine permits fuel intakedirectly into the crankcase. See Figure 5-14. Thereed is made of thin, flexible spring steel, which isfastened at one end. See Figure 5-15. The oppo-site end covers the intake port. The reed stop isthick and inflexible. It prevents the reed from

opening too far and becoming permanently bent.

In operation, the reed is opened by atmos-pheric pressure during the intake stroke. It isclosed by the springiness of the metal and thecompression in the crankcase on the power stroke.Figure 5-16A illustrates the air-fuel mixtureentering the crankcase. Figure 5-16B shows howthe reed valve is closed by crankcase pressure.

There are many reed valve designs. Some typ-

ical configurations are illustrated in Figure 5-17.

Chapter 5 Two-Cycle and Four-Cycle Engines 89

Figure 5-11. Pressure pulse exhaust tuning is an effective way of increasing power and efficiency in two-cycle

engines. Exhaust sound waves reflected back into the manifold create a back pressure that stops the fuel mixture

from leaving the cylinder before the piston closes the port. This system requires precise engineering. (Kohler Co.)

,

Figure 5-12. A straight pipe may sound louder and more powerful than tuned exhaust, but actually is far less efficient.In this illustration, center of sound is too far and lacks amplification to have any beneficial effect on engine.

90 Small Gas Engines


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Four-Cycle Engine vs.Two-Cycle Engine

The advantages and disadvantages of any

engine are directly related to the purpose for which

the engine is intended. It cannot be said that onetype of engine is better than another without con-sidering every aspect of its application.

The chart in Figure 5-18 lists the differencesbetween two- and four-cycle engines.

Summary

The stroke of a piston is its movement in thecylinder from one end of its travel to another.Four-stroke cycle engines need four strokes tocomplete the operating cycle: intake, compression,power, and exhaust. Lubrication of four-cycleengines is generally provided by a splash system

or a pump system.In a two-stroke cycle engine, the intake, com-pression, power, and exhaust functions take placeduring two strokes of the piston. Two-cycleengines have many advantages over four-cycleunits. They do not have conventional valves, tap-pets, or a camshaft, so they are simpler in design.Two-cycle engines are also smaller and lighterthan four-cycle engines of equivalent horsepower.

The two-cycle engine receives its lubricationas a fuel-oil mixture is passed through the engine.Therefore, it will receive adequate lubrication

even when operated at extreme angles.

Chapter 5 Two-Cycle and Four-Cycle Engines 91

Figure 5-14. A popular method of crankcase valving is a reed valve designed to fit into the crankcase wall. It relies

upon the difference between atmospheric pressure and crankcase pressure to be opened.The default position of

the reed valve is the closed position.

Figure 5-15. This side view of a reed valve shows the

spring steel reed covering the entry hole.The reed stop

controls the distance the reed may open. This prevents

permanent distortion and failure of the reed to return

snugly against the port during crankcase compression.

A B C

92 Small Gas Engines

Figure 5-17. Several forms of reed valves. ASingle reed, closed position. BSingle reed, open position. Note

the reed opening distance is controlled by the stop. CTwin reed. DTriple reed. EAnother form of triple ree

FSingle reed. GMultiple reed. HFour reed. IMultiple reed.

Figure 5-16. Reed valve action. AVacuum in the crankcase, formed by the upward moving piston, causesatmospheric pressure to force air-fuel mixture through the port opening. BDownward piston movement com-

presses the fuel mixture in the crankcase to a pressure greater than atmospheric pressure. The springiness of

reed and crankcase pressure act together to close the port.

A B


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2stroke Engine vs 4 Stroke Engine

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